Displacement sensors such as LVDTs and RVDTs are commonly used in various applications, such a actuators connected to aero-space vehicle control surfaces, machine tools and the like, for movement detection. A displacement sensor generally has a primary coil and two secondary coils. The coils are wound on a bobbin wherein a movable slug is enclosed. The primary coil receives an excitation signal which induces mutually opposite currents in the two secondary coils. Because the position of the slug affects the inductance between the primary coil and each of the secondary coils, the currents induced in the secondary coils by the excitation signal can thus be used to detect position of the slug.
Unfortunately, the currents induced in the secondary coils are also dependent upon the frequency and amplitude of the excitation signal. If either the amplitude or the frequency of the excitation signal changes, the output of the sensors would change even when the slug remains stationary. Therefore, to operate properly, conventional displacement sensors generally require an excitation signal with stable amplitude and frequency.
Operability and cost of sensors would greatly improve if a stable excitation signal is not required. For example, if there is no requirement of a stable excitation signal, the sensor can then be operated with the same excitation signal which is used to drive the LVDT of an actuator can also be used to drive a hydraulic valve's LVDT, thus eliminating the possibility of "beat" frequencies which could occur when two different excitation signals are used.
Therefore, what is needed is a displacement sensor which can provide accurate displacement information without requiring a stable excitation signal.
Moreover, in applications such as a high performance control loop, information on velocity and acceleration of the slug, in addition to its displacement, are sometimes required for stabilization purposes. Unfortunately, to provide velocity and acceleration information, conventional displacement sensors either require additional transducers, or a slug that has both high permeability and magnetic retentability.
Therefore, what is also needed is a displacement sensor which can provide velocity and/or acceleration information without the need of additional transducers and without the need of a special slug.